Load transfer plate for in situ concrete slabs

ABSTRACT

A tapered load plate transfers loads across a joint between adjacent concrete floor slabs. The top and bottom surfaces may taper from approximately 4 inches wide to a narrow substantially pointed end over a length of approximately 12 inches. The tapered load plate accommodates differential shrinkage of cast-in-place concrete slabs. When adjacent slabs move away from each other, the narrow end of the tapered load plate moves out of the void that it created in the slab thus allowing the slabs to move relative to one another in a direction parallel to the joint. Tapered load plates may be assembled into a load-plate basket with the direction of the taper alternating from one tapered load plate to the next to account for off-center saw cuts. A tapered load plate and an end cap may be used to provide load transfer across an expansion joint.

This application claims priority to provisional U.S. Application Ser.No. 60/318,838, filed Sep. 13, 2001.

TECHNICAL FIELD

This invention relates generally to transferring loads between adjacentcast-in-place slabs and more particularly to a system for transferring,across a joint between a first slab and a second slab, a load applied toeither slab.

BACKGROUND OF THE INVENTION

Referring to FIG. 1, when a concrete floor slab 100 is first placed andthe concrete starts to cure the volume of the concrete decreases causingthe slab to shrink (usually on the order of ⅛ of an inch per 20 feet).Concrete has a relatively low strength when in tension. When theinternal stresses due to shrinkage 104 reach a point greater then thetensile strength of the concrete, random stress-relief cracks 102 occur.

These random cracks 102 are undesirable as they detract from theperformance of the floor slab 100 and reduce its life span. Referring toFIGS. 2A and 2B, a typical method of controlling where these cracks 102occur is to induce a weakened plane by saw cutting the top surface 200of the concrete slab 100 into small panels, as depicted by saw cut 202.

Referring to FIG. 3, an undesirable side effect of having the floor slab100 made up of numerous small sections is that when the floor is loaded,such as with the wheels of a moving fork lift 300, each section of thefloor may be deflected 302 relative to its neighbor causing damage 304to the joint edge, as depicted in FIG. 3.

Referring to FIG. 4, a conventional technique for reducing this type ofdeflection 302 is to span the joint 400 with steel bars 402 each havinga round cross-section. These bars 402 are commonly referred to as dowelbars.

Referring to FIGS. 5A-5C, dowels of this type are typically assembledinto a wirework frame 500 that holds the dowels at a desired depth 502and orientation. This assembly is generally known as a dowel basket.

Using circular-cross-section dowel bars is associated with variousdrawbacks. For instance, if the dowel bars 402 are misaligned 600 suchthat they are not oriented totally perpendicular to the joint, the dowelbars 402 can lock the joint 400 thereby undesirably restraining thejoint from opening, which in turn may cause random cracks 102.

Referring to FIG. 7, if a concrete floor slab, such as slabs 100-1 or100-2, tries to move along the line of the joint 400 relative to thenext panel (for instance due to shrinkage or thermal contraction), thedowel bars 402 will restrain this type of movement 700, thereby causingrandom cracks 102.

Referring to FIG. 8, at an intersection of two joints, movement 800,which is a combination of the two types of movement discussed above inconnection with FIGS. 6 and 7, can cause a situation known as cornercracking 802.

Referring to FIGS. 9A and 9B, the round-dowel-bar drawbacks discussedabove have been addressed in the past by using dowel bars 900 having asquare or rectangular cross-section in conjunction with a plastic orsteel clip 902 that places a compressible material 904 on the twovertical faces of the dowel bar 900. These clips 902 produce a void inthe concrete wider than the dowel bar 900 allowing for sideways movementand a slight degree of misalignment. The clips 902, however, undesirablyadd to the expense associated with using dowel bars 900 having squareand/or rectangular cross-sections. A more cost-effective solution thatovercomes the misalignment problem to a greater extent, therefore, wouldbe advantageous.

Under certain conditions, such as outdoor applications, concrete slabplacement should be able to withstand concrete expansion, which istypically due to thermal changes, such as colder winter temperatureschanging to warmer summer temperatures. Referring to FIG. 10,conventionally, a piece of compressible material 1000, such as foam,fiberboard, timber, or the like, is placed in an expansion joint 1002between concrete slabs 100-1 and 100-2. A round-cross-section dowel bar402 and an end cap 1004 may be used for transferring a load across theexpansion joint 1002. As the slabs 100 expand, they move together, asindicated by arrows 1006, the joint 1002 closes, and the dowel bar 402goes farther into the end cap 1004. This use of round-cross-sectiondowel bars, however, is associated with the misalignment drawbackdiscussed above in connection with saw-cut control joints. Acost-effective way of dealing with the misalignment situation whiletransferring loads between concrete slabs across expansion joints 1002would therefore be desirable.

Applicants' U.S. Pat. No. 6,354,760 discloses a load plate thatovercomes the drawbacks discussed above, namely misalignment andallowing relative movement of slabs parallel to the joint. Referring toFIG. 11, the '760 patent discloses using a load plate 1100 rotated suchthat the load plate has a widest portion (i.e., opposite corners) of theload plate positioned in the joint between slabs 100-1 and 100-2. Usingsuch a load plate 1100 at a construction joint works well because theload plate can be reliably centered at the construction joint betweenthe slabs 100.

A load plate 1100 is not, however, ideally suited for use at saw-cutcontrol joints. As described above, this type of joint results fromcracking induced by a saw cut in the upper surface of a concrete slab.The saw cut may be off center with respect to any load plate embeddedwithin the cement, as shown by the dashed line 1200 in FIG. 12. If thesaw cut and joint are off-center, the load plate will not function asintended because more than half of the load plate will be fixed withinone of the slabs and less than half of the load plate will be availablefor transferring loads to and from the other slab. Another situation forwhich a load plate 1100 is not ideally suited is when a constructionjoint, formed by an edge form, for instance, is expected to berelatively wide open. Under such circumstances, an undesirably largearea of load plates 1100 may undesirably be removed from slabs on eitheror both sides of the joint thereby reducing the ability of the loadplate 1100 to transfer loads between the slabs. For these reasons, aload transfer device that provides the advantages of the load plate ofthe '760 patent and that is well suited to use in saw-cut control jointsand construction joints, which may become relatively wide open, would bedesirable.

SUMMARY OF THE INVENTION

In accordance with an illustrative embodiment of the invention, atapered load plate may be used to transfer loads across a joint betweenadjacent concrete floor slabs. The top and bottom surfaces may taperfrom approximately 4 inches wide to a narrow substantially pointed end1308 over a length of approximately 12 inches. As will be apparent,other suitable tapered shapes and/or other suitable dimensions may alsobe used.

A tapered load plate, in accordance with an illustrative embodiment ofthe invention, advantageously accommodates misalignment of a saw cut forcreating a control joint. Misalignment up to an angle substantiallyequal to the angle of the load plate's taper may be accommodated.

The tapered shape of the tapered load plate advantageously accommodatesdifferential shrinkage of cast-in-place concrete slabs. When adjacentslabs move away from each other, the narrow end of the tapered loadplate moves out of the void that it created in the slab. As the taperedload plate retracts, it will occupy less space within the void in theslab thus allowing the slabs to move relative to one another in adirection parallel to the joint.

Tapered load plates may be assembled into a load-plate basket with thedirection of the taper alternating from one tapered load plate to thenext. If a saw cut, used for creating a control joint, is positionedoff-center relative to the tapered load plates, the alternating patternof tapered load plates in the load-plate basket will ensure that thecross section of tapered load plate material, such as steel, spanningthe joint remains substantially constant across any number of pairs oftapered load plates. For use in connection with a construction joint, anedge form may be used to position tapered load plates before the slabsare cast in place.

In accordance with an illustrative embodiment of the invention, atapered load plate and an end cap, may be used to provide load transferacross an expansion joint. The tapered shape of the load plate willallow for misalignment. As either or both slabs expand and thereby causethe joint to close, the wide end of the tapered load plate moves fartherinto the end cap. This results in the allowance of an increasing amountof lateral movement between the slabs parallel to the joint 400 to thecentral and relatively wider portions of the tapered load plateoccupying less space in the tapered void.

In accordance with an illustrative embodiment of the invention, atapered-load-plate basket may be used to position the tapered loadplates and compressible material before the concrete slabs are cast inplace.

Additional features and advantages of the invention will be apparentupon reviewing the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a concrete floor slab with random cracks causedby concrete shrinkage.

FIGS. 2A and 2B are cross-section and plan views of saw-cut controljoints.

FIG. 3 depicts vertical deflection of a floor slab under a load anddamage to an adjacent floor slab.

FIGS. 4A and 4B are cross section and plan view of dowel bars positionedfor transferring loads across joints between adjacent slabs.

FIGS. 5A-5C are plan and sectional views of a dowel basket forpositioning dowel bars before a floor slab is cast in place.

FIG. 6 is a plan view of misaligned dowel bars locking a joint andthereby causing a slab to crack.

FIG. 7 is a plan view of cracks caused by dowel bars restrictingrelative movement of slabs parallel to the joint between the slabs.

FIG. 8 is a plan view showing corner cracking due to misaligned dowelbars and restricted relative movement of slabs parallel to the joints.

FIGS. 9A and 9B are isometric and sectional views of a square dowel andsquare-dowel clip.

FIG. 10 is a side view of a typical expansion joint with compressiblematerial in the joint.

FIG. 11 is a plan view of a diamond-shaped load plate between two slabs.

FIG. 12 is a plan view illustrating an off-center saw cut relative todiamond-shaped load plates.

FIG. 13 shows a top and two side views of a tapered load plate inaccordance with an illustrative embodiment of the invention.

FIG. 14 is a plan view showing a misaligned saw cut relative to atapered load plate.

FIG. 15 is a plan view of a tapered load plate, two slabs, a joint, anda void created by the narrow end of the tapered load plate.

FIG. 16 shows tapered load plates in a tapered-load-plate basket,wherein the orientation of the tapered load plates alternates from onetapered load plate to the next.

FIG. 17 is a plan view showing an off-center saw cut relative to threealternately oriented tapered load plates.

FIG. 18 is a plan view of an open expansion joint, a tapered load plate,and an end cap.

FIG. 19 is a plan view similar to FIG. 18 with the joint having closedrelative to FIG. 18.

FIG. 20 is a side view of an expansion-type tapered-load-plate basket,compressible material, a tapered load plate, and an end cap.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 13, in accordance with an illustrative embodiment ofthe invention, a tapered load plate, such as tapered load plate 1300,may be used to transfer loads across a joint between adjacent concretefloor slabs. The tapered load plate 1300 may have top and bottomsurfaces that are tapered, substantially planar, and substantiallyparallel to one another. A triangular-shaped tapered top surface 1302and two generally rectangular-shaped side surfaces 1304 and 1306 areshown in FIG. 13. The top and bottom surfaces may taper fromapproximately 4 inches wide to a narrow substantially pointed end 1308over a length of approximately 12 inches. As will be apparent, othersuitable tapered shapes and/or other suitable dimensions may also beused.

A tapered load plate 1300, in accordance with an illustrative embodimentof the invention, advantageously accommodates misalignment of a saw cutfor creating a control joint. Misalignment up to an angle substantiallyequal to the angle of the load plate's taper may be accommodated.Referring to FIG. 14, a misaligned saw cut 1400 is misaligned by anangle 1402 from correctly aligned saw cut 1404, which is orientedperpendicular to the tapered load plate's longitudinal axis 1406. Theload plate's angle of taper is depicted in FIG. 14 by angle 1408.

Referring to FIG. 15, differential shrinkage of cast-in-place concreteslabs is advantageously accommodated by the tapered shape of the taperedload plate 1300. When adjacent slabs, such as slabs 100-1 and 100-2,move away from each other, as indicated by arrow 1500, the joint 400 issaid to open. As this occurs, the narrow end of the tapered load plate1300 moves out of the void 1502 that it created in the slab 100-2. Asthe tapered load plate 1300 retracts in this manner, it will occupy lessspace within the void in the slab 100-2 thus allowing the slabs 100-1and 100-2 to move relative to one another in a direction parallel to thejoint 400. In other words, as the slabs move apart, the narrow end ofthe tapered load plate occupies less of the width of the tapered void1502.

Referring to FIG. 16, tapered load plates 1300 may be assembled into aload-plate basket 1600 with the direction of the taper alternating fromone tapered load plate 1300 to the next. Referring to FIG. 17, if a sawcut 1700, used for creating a control joint, is positioned off-centerrelative to the tapered load plates 1300, the alternating pattern oftapered load plates 1300 in the load-plate basket 1600 will ensure thatthe cross section of tapered load plate material, such as steel,spanning the joint remains substantially constant across any number ofpairs of tapered load plates 1300. For use in connection with aconstruction joint an edge form may be used to position tapered loadplates before the slabs are cast in place.

Referring to FIG. 18, in accordance with an illustrative embodiment ofthe invention, a tapered load plate 1300 and an end cap 1800 may be usedto provide load transfer across an expansion joint of the type discussedabove in connection with FIG. 10. The tapered shape of the load plate1300 will allow for misalignment, as discussed above in connection withFIG. 14. As either or both slabs 100-1 and 100-2 expand and therebycause the joint 400 to close, the wide end of the tapered load plate1300 moves farther into the end cap 1800. This results in the allowanceof an increasing amount of lateral movement between the slabs 100-1 and100-2 parallel to the joint 400 due to the central and relatively widerportions of the tapered load plate occupying less space in the taperedvoid 1900.

Referring to FIG. 20, in accordance with an illustrative embodiment ofthe invention, a tapered-load-plate basket 2000 may be used to positionthe tapered load plates 1300 and compressible material 1000 before theconcrete slabs 100 are cast in place.

While the invention has been described with respect to specific examplesincluding presently preferred modes of carrying out the invention, theinvention is limited only by the following claims.

1. A system for transferring loads across a joint between concreteon-ground cast-in-place slabs, the system comprising: a first concreteon-ground cast-in-place slab; a second concrete on-ground cast-in-placeslab; an expansion joint separating the first and second slabs, whereinthe joint is oriented substantially perpendicular to a substantiallyplanar upper surface of the first slab, and a longitudinal axis of thejoint is formed by an intersection of the joint and the upper surface ofthe first slab; a load-plate end cap embedded within the first slab; atapered load plate having a width measured in a direction substantiallyparallel to said longitudinal axis, and having only one relatively wideportion and only one relatively narrow portion, that tapers from saidrelatively wide portion, said taper from said relatively wide portionbeing a generally progressive reduction of said width of said load plateas said load plate extends from said wide portion across said expansionjoint, said taper including said generally progressive reduction of saidwidth continuing past said expansion joint as said load plate extends tosaid relatively narrow portion, the wide portion protruding into saidfirst slab and a portion of the end cap, and the narrow end protrudinginto the second slab, such that the load plate transfers between thefirst and second slabs a load applied to either of the slabs directedsubstantially perpendicular to the upper surface of the first slab; andwhereby the load plate restricts relative movement between the first andsecond slabs in a direction substantially perpendicular to the uppersurface of the first slab, and the load plate moves farther into the endcap as the joint closes via the first and second slabs moving towardeach other in a direction substantially perpendicular to the joint. 2.The system of claim 1, further comprising: a second load-plate end capembedded within the second slab; a second tapered load plate having awidth measured in a direction substantially parallel to saidlongitudinal axis, and having only one relatively wide portion and onlyone relatively narrow portion, that tapers from said relatively wideportion, said taper from said relatively wide portion of said secondplate being a generally progressive reduction of said width of saidsecond load plate as said second load plate extends from said secondload plate wide portion across said expansion joint, said taperincluding said generally progressive reduction of said width continuingpast said expansion joint as said second load plate extends to saidrelatively narrow portion, the wide portion protruding into said secondslab and a portion of the second end cap, and the narrow portionprotruding into the first slab, such that the load plate transfersbetween the first and second slabs a load applied to either of the slabsdirected substantially perpendicular to the upper surface of the firstslab; and whereby the second load plate restricts relative movementbetween the first and second slabs in a direction substantiallyperpendicular to the upper surface of the first slab, and the secondload plate moves farther into the second end cap as the joint closes viathe first and second slabs moving toward each other in a directionsubstantially perpendicular to the joint.
 3. The system of claim 2,wherein the tapered load plates each have a length measuredperpendicular to the joint that is substantially greater than the wideportions.
 4. The system of claim 2, wherein the tapered load plates'wide portions are wide ends.
 5. The system of claim 4, wherein thetapered load plates' narrow ends taper to respective substantiallypointed ends.
 6. The system of claim 2, further comprising atapered-load-plate basket that positions the tapered load plates beforethe slabs are cast in place.
 7. A system for restricting certainmovement, accommodating certain other movement and transferring loadsbetween a first concrete on-ground cast-in-place slab and a secondconcrete on-ground cast-in-place slab, the system comprising the slabsand further comprising: a joint interposing the first and second slabs,at least the first slab having a substantially planar upper surface, atleast a portion of the joint being initially defined by at least one ofa crack, cut or a form oriented substantially perpendicular to thesubstantially planar upper surface of the first slab, wherein alongitudinal axis of the joint is formed by an intersection of the cutor form and the upper surface of the first slab and wherein the joint issubject to opening through a range of joint opening dimensions andbeyond; a first tapered load plate and a second tapered load plate thateach have a taper, protrude into the first and second slabs and have anextent across the joint such that the load plates span the joint andtransfer between the first and second slabs a load applied to either ofthe slabs directed substantially perpendicular to the upper surface ofthe first slab; the tapered load plates each having a width measuredparallel to the longitudinal axis of the joint; the width of eachtapered load plate generally tapering from a relatively wide location inthe extent of each plate across the joint to a relatively narrow portionsuch that, as the joint opens, a tapered gap opens between the loadplate and the slab near the narrow end portion such that the slabs areallowed increasingly greater relative movement in the directionsubstantially parallel to the longitudinal axis of the joint; andwherein the first and second tapered load plates are oriented such thatfor at least the range of joint opening dimensions, reduced width of oneload plate at the narrowest width in the joint of the one load plate dueto plate taper is compensated for by increased width of the other loadplate in the joint due to opposing plate taper, such that for at leastthe range of joint opening dimensions, the combined widths of the firstand second tapered load plates in the joint is consistently adequate forload transfer across the joint; whereby the tapered load plates restrictrelative movement between the first and second slabs in a directionsubstantially perpendicular to the upper surface of the first slab,allow the joint to open by allowing the first and second slabs to moveaway from each other in a direction substantially perpendicular to thejoint, allow for increasingly greater relative movement in a directionsubstantially parallel to the longitudinal axis of the joint as thejoint opens, and maintain consistently adequate load transfer across thejoint.
 8. The system of claim 7, wherein the tapered load plates eachhave a length measured perpendicular to the joint that is substantiallygreater than the wide portions.
 9. The system of claim 7, wherein: thetapered load plates' wide portions are wide ends; and the tapered loadplates' narrow portions staper to respective substantially pointed ends.10. The system of claim 7, further comprising a tapered-load-platebasket that positions the tapered load plates before the slabs are castin place.
 11. The system of claim 7 or 10, wherein the joint is asaw-cut control joint.
 12. The system of claim 11, wherein the firsttapered load plate's wide portion protrudes into the first slab and thesecond tapered load plate's wide portion protrudes into the second slab.13. A system for transferring loads between a first concrete on-groundcast-in-place slab and a second concrete on-ground cast-in-place slab,the system comprising: a joint separating the first and second slabs, atleast a portion of the joint being initially defined by a partial depthsaw cut that results in a crack below the saw cut, wherein alongitudinal axis of the joint is formed by an intersection of the sawcut and the upper surface of the first slab; a first load plate and asecond load plate that each protrude into the first and second slabssuch that the load plates transfer between the first and second slabs aload applied to either of the slabs directed substantially perpendicularto the upper surface of the first slab; whereby the load plates restrictrelative movement between the first and second slabs in a directionsubstantially perpendicular to the upper surface of the first slab, andthe load plates allow the joint to open by allowing the first and secondslabs to move away from each other in a direction substantiallyperpendicular to the joint; the load plates each having a width measuredparallel to the longitudinal axis of the joint; and wherein the width ofeach load plate generally tapers from a relatively wide portion near thejoint to at least one relatively narrow end in at least one of the slabssuch that, as the joint opens, the slabs are allowed increasinglygreater relative movement in a direction substantially parallel to thelongitudinal axis of the joint; and wherein the tapered load platesdefine a cross section of tapered load plate material spanning thejoint, and the cross section remains substantially constant between thesaw cut being positioned on-center relative to the tapered load platesand the saw cut being, in at least one position of the saw cut,off-center relative to the tapered load plates.
 14. The system of claim13, wherein the load plates taper to respective substantially pointedends.
 15. The system of claim 13, further comprising a load-plate basketthat positions the load plates before the slabs are cast in place. 16.The system of claim 13, wherein the first load plate's relatively narrowend protrudes into the first slab and the second load plate's relativelynarrow end protrudes into the second slab.
 17. The system of claim 13,wherein the width of each load plate generally tapers from a relativelywide end to the relatively narrow end.
 18. The system of claim 17,wherein the first relatively narrow end tapers to a first substantiallypointed end.
 19. The system of claim 18, wherein the second relativelynarrow end tapers to a second substantially pointed end.